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Real World Haskell

Cover of Real World Haskell by John Goerzen... Published by O'Reilly Media, Inc.
  1. Real World Haskell
    1. SPECIAL OFFER: Upgrade this ebook with O’Reilly
    2. A Note Regarding Supplemental Files
    3. Preface
      1. Have We Got a Deal for You!
      2. What to Expect from This Book
      3. What to Expect from Haskell
      4. A Brief Sketch of Haskell’s History
      5. Helpful Resources
      6. Conventions Used in This Book
      7. Using Code Examples
      8. Safari® Books Online
      9. How to Contact Us
      10. Acknowledgments
    4. 1. Getting Started
      1. Your Haskell Environment
      2. Getting Started with ghci, the Interpreter
      3. Basic Interaction: Using ghci as a Calculator
      4. Command-Line Editing in ghci
      5. Lists
      6. Strings and Characters
      7. First Steps with Types
      8. A Simple Program
    5. 2. Types and Functions
      1. Why Care About Types?
      2. Haskell’s Type System
      3. What to Expect from the Type System
      4. Some Common Basic Types
      5. Function Application
      6. Useful Composite Data Types: Lists and Tuples
      7. Functions over Lists and Tuples
      8. Function Types and Purity
      9. Haskell Source Files, and Writing Simple Functions
      10. Understanding Evaluation by Example
      11. Polymorphism in Haskell
      12. The Type of a Function of More Than One Argument
      13. Why the Fuss over Purity?
      14. Conclusion
    6. 3. Defining Types, Streamlining Functions
      1. Defining a New Data Type
      2. Type Synonyms
      3. Algebraic Data Types
      4. Pattern Matching
      5. Record Syntax
      6. Parameterized Types
      7. Recursive Types
      8. Reporting Errors
      9. Introducing Local Variables
      10. The Offside Rule and Whitespace in an Expression
      11. The case Expression
      12. Common Beginner Mistakes with Patterns
      13. Conditional Evaluation with Guards
    7. 4. Functional Programming
      1. Thinking in Haskell
      2. A Simple Command-Line Framework
      3. Warming Up: Portably Splitting Lines of Text
      4. Infix Functions
      5. Working with Lists
      6. How to Think About Loops
      7. Anonymous (lambda) Functions
      8. Partial Function Application and Currying
      9. As-patterns
      10. Code Reuse Through Composition
      11. Tips for Writing Readable Code
      12. Space Leaks and Strict Evaluation
    8. 5. Writing a Library: Working with JSON Data
      1. A Whirlwind Tour of JSON
      2. Representing JSON Data in Haskell
      3. The Anatomy of a Haskell Module
      4. Compiling Haskell Source
      5. Generating a Haskell Program and Importing Modules
      6. Printing JSON Data
      7. Type Inference Is a Double-Edged Sword
      8. A More General Look at Rendering
      9. Developing Haskell Code Without Going Nuts
      10. Pretty Printing a String
      11. Arrays and Objects, and the Module Header
      12. Writing a Module Header
      13. Fleshing Out the Pretty-Printing Library
      14. Creating a Package
      15. Practical Pointers and Further Reading
    9. 6. Using Typeclasses
      1. The Need for Typeclasses
      2. What Are Typeclasses?
      3. Declaring Typeclass Instances
      4. Important Built-in Typeclasses
      5. Automatic Derivation
      6. Typeclasses at Work: Making JSON Easier to Use
      7. Living in an Open World
      8. How to Give a Type a New Identity
      9. JSON Typeclasses Without Overlapping Instances
      10. The Dreaded Monomorphism Restriction
      11. Conclusion
    10. 7. I/O
      1. Classic I/O in Haskell
      2. Working with Files and Handles
      3. Extended Example: Functional I/O and Temporary Files
      4. Lazy I/O
      5. The IO Monad
      6. Is Haskell Really Imperative?
      7. Side Effects with Lazy I/O
      8. Buffering
      9. Reading Command-Line Arguments
      10. Environment Variables
    11. 8. Efficient File Processing, Regular Expressions, and Filename Matching
      1. Efficient File Processing
      2. Filename Matching
      3. Regular Expressions in Haskell
      4. More About Regular Expressions
      5. Translating a glob Pattern into a Regular Expression
      6. An important Aside: Writing Lazy Functions
      7. Making Use of Our Pattern Matcher
      8. Handling Errors Through API Design
      9. Putting Our Code to Work
    12. 9. I/O Case Study: A Library for Searching the Filesystem
      1. The find Command
      2. Starting Simple: Recursively Listing a Directory
      3. A Naive Finding Function
      4. Predicates: From Poverty to Riches, While Remaining Pure
      5. Sizing a File Safely
      6. A Domain-Specific Language for Predicates
      7. Controlling Traversal
      8. Density, Readability, and the Learning Process
      9. Another Way of Looking at Traversal
      10. Useful Coding Guidelines
    13. 10. Code Case Study: Parsing a Binary Data Format
      1. Grayscale Files
      2. Parsing a Raw PGM File
      3. Getting Rid of Boilerplate Code
      4. Implicit State
      5. Introducing Functors
      6. Writing a Functor Instance for Parse
      7. Using Functors for Parsing
      8. Rewriting Our PGM Parser
      9. Future Directions
    14. 11. Testing and Quality Assurance
      1. QuickCheck: Type-Based Testing
      2. Testing Case Study: Specifying a Pretty Printer
      3. Measuring Test Coverage with HPC
    15. 12. Barcode Recognition
      1. A Little Bit About Barcodes
      2. Introducing Arrays
      3. Encoding an EAN-13 Barcode
      4. Constraints on Our Decoder
      5. Divide and Conquer
      6. Turning a Color Image into Something Tractable
      7. What Have We Done to Our Image?
      8. Finding Matching Digits
      9. Life Without Arrays or Hash Tables
      10. Turning Digit Soup into an Answer
      11. Working with Row Data
      12. Pulling It All Together
      13. A Few Comments on Development Style
    16. 13. Data Structures
      1. Association Lists
      2. Maps
      3. Functions Are Data, Too
      4. Extended Example: /etc/passwd
      5. Extended Example: Numeric Types
      6. Taking Advantage of Functions as Data
      7. General-Purpose Sequences
    17. 14. Monads
      1. Revisiting Earlier Code Examples
      2. Looking for Shared Patterns
      3. The Monad Typeclass
      4. And Now, a Jargon Moment
      5. Using a New Monad: Show Your Work!
      6. Mixing Pure and Monadic Code
      7. Putting a Few Misconceptions to Rest
      8. Building the Logger Monad
      9. The Maybe Monad
      10. The List Monad
      11. Desugaring of do Blocks
      12. The State Monad
      13. Monads and Functors
      14. The Monad Laws and Good Coding Style
    18. 15. Programming with Monads
      1. Golfing Practice: Association Lists
      2. Generalized Lifting
      3. Looking for Alternatives
      4. Adventures in Hiding the Plumbing
      5. Separating Interface from Implementation
      6. The Reader Monad
      7. A Return to Automated Deriving
      8. Hiding the IO Monad
    19. 16. Using Parsec
      1. First Steps with Parsec: Simple CSV Parsing
      2. The sepBy and endBy Combinators
      3. Choices and Errors
      4. Extended Example: Full CSV Parser
      5. Parsec and MonadPlus
      6. Parsing a URL-Encoded Query String
      7. Supplanting Regular Expressions for Casual Parsing
      8. Parsing Without Variables
      9. Applicative Functors for Parsing
      10. Applicative Parsing by Example
      11. Parsing JSON Data
      12. Parsing a HTTP Request
    20. 17. Interfacing with C: The FFI
      1. Foreign Language Bindings: The Basics
      2. Regular Expressions for Haskell: A Binding for PCRE
      3. Passing String Data Between Haskell and C
      4. Matching on Strings
    21. 18. Monad Transformers
      1. Motivation: Boilerplate Avoidance
      2. A Simple Monad Transformer Example
      3. Common Patterns in Monads and Monad Transformers
      4. Stacking Multiple Monad Transformers
      5. Moving Down the Stack
      6. Understanding Monad Transformers by Building One
      7. Transformer Stacking Order Is Important
      8. Putting Monads and Monad Transformers into Perspective
    22. 19. Error Handling
      1. Error Handling with Data Types
      2. Exceptions
      3. Error Handling in Monads
    23. 20. Systems Programming in Haskell
      1. Running External Programs
      2. Directory and File Information
      3. Program Termination
      4. Dates and Times
      5. Extended Example: Piping
    24. 21. Using Databases
      1. Overview of HDBC
      2. Installing HDBC and Drivers
      3. Connecting to Databases
      4. Transactions
      5. Simple Queries
      6. SqlValue
      7. Query Parameters
      8. Prepared Statements
      9. Reading Results
      10. Database Metadata
      11. Error Handling
    25. 22. Extended Example: Web Client Programming
      1. Basic Types
      2. The Database
      3. The Parser
      4. Downloading
      5. Main Program
    26. 23. GUI Programming with gtk2hs
      1. Installing gtk2hs
      2. Overview of the GTK+ Stack
      3. User Interface Design with Glade
      4. Event-Driven Programming
      5. Initializing the GUI
      6. The Add Podcast Window
      7. Long-Running Tasks
      8. Using Cabal
    27. 24. Concurrent and Multicore Programming
      1. Defining Concurrency and Parallelism
      2. Concurrent Programming with Threads
      3. Simple Communication Between Threads
      4. The Main Thread and Waiting for Other Threads
      5. Communicating over Channels
      6. Useful Things to Know About
      7. Shared-State Concurrency Is Still Hard
      8. Using Multiple Cores with GHC
      9. Parallel Programming in Haskell
      10. Parallel Strategies and MapReduce
    28. 25. Profiling and Optimization
      1. Profiling Haskell Programs
      2. Controlling Evaluation
      3. Understanding Core
      4. Advanced Techniques: Fusion
    29. 26. Advanced Library Design: Building a Bloom Filter
      1. Introducing the Bloom Filter
      2. Use Cases and Package Layout
      3. Basic Design
      4. The ST Monad
      5. Designing an API for Qualified Import
      6. Creating a Mutable Bloom Filter
      7. The Immutable API
      8. Creating a Friendly Interface
      9. Creating a Cabal Package
      10. Testing with QuickCheck
      11. Performance Analysis and Tuning
    30. 27. Sockets and Syslog
      1. Basic Networking
      2. Communicating with UDP
      3. Communicating with TCP
    31. 28. Software Transactional Memory
      1. The Basics
      2. Some Simple Examples
      3. STM and Safety
      4. Retrying a Transaction
      5. Choosing Between Alternatives
      6. I/O and STM
      7. Communication Between Threads
      8. A Concurrent Web Link Checker
      9. Practical Aspects of STM
    32. A. Installing GHC and Haskell Libraries
      1. Installing GHC
      2. Installing Haskell Software
    33. B. Characters, Strings, and Escaping Rules
      1. Writing Character and String Literals
      2. International Language Support
      3. Escaping Text
    34. Index
    35. About the Authors
    36. Colophon
    37. SPECIAL OFFER: Upgrade this ebook with O’Reilly

Partial Function Application and Currying

You may wonder why the -> arrow is used for what seems to be two purposes in the type signature of a function:

ghci> :type dropWhile
dropWhile :: (a -> Bool) -> [a] -> [a]

It looks like the -> is separating the arguments to dropWhile from each other, but that it also separates the arguments from the return type. Iin fact -> has only one meaning: it denotes a function that takes an argument of the type on the left and returns a value of the type on the right.

The implication here is very important. In Haskell, all functions take only one argument. While dropWhile looks like a function that takes two arguments, it is actually a function of one argument, which returns a function that takes one argument. Here’s a perfectly valid Haskell expression:

ghci> :module +Data.Char
ghci> :type dropWhile isSpace
dropWhile isSpace :: [Char] -> [Char]

Well, that looks useful. The value dropWhile isSpace is a function that strips leading whitespace from a string. How is this useful? As one example, we can use it as an argument to a higher order function:

ghci> map (dropWhile isSpace) [" a","f","   e"]

Every time we supply an argument to a function, we can chop an element off the front of its type signature. Let’s take zip3 as an example to see what we mean; this is a function that zips three lists into a list of three-tuples:

ghci> :type zip3
zip3 :: [a] -> [b] -> [c] -> [(a, b, c)]
ghci> zip3 "foo" "bar" "quux"

If we apply zip3 with just one argument, we get a function that accepts two arguments. No matter what arguments we supply to this compound function, its first argument will always be the fixed value we specified:

ghci> :type zip3 "foo"
zip3 "foo" :: [b] -> [c] -> [(Char, b, c)]
ghci> let zip3foo = zip3 "foo"
ghci> :type zip3foo
zip3foo :: [b] -> [c] -> [(Char, b, c)]
ghci> (zip3 "foo") "aaa" "bbb"
ghci> zip3foo "aaa" "bbb"
ghci> zip3foo [1,2,3] [True,False,True]

When we pass fewer arguments to a function than the function can accept, we call it partial application of the function—we’re applying the function to only some of its arguments.

In the previous example, we have a partially applied function, zip3 "foo", and a new function, zip3foo. We can see that the type signatures of the two and their behavior are identical.

This applies just as well if we fix two arguments, giving us a function of just one argument:

ghci> let zip3foobar = zip3 "foo" "bar"
ghci> :type zip3foobar
zip3foobar :: [c] -> [(Char, Char, c)]
ghci> zip3foobar "quux"
ghci> zip3foobar [1,2]

Partial function application lets us avoid writing tiresome throwaway functions. It’s often more useful for this purpose than the anonymous functions we introduced earlier in this chapter in Anonymous (lambda) Functions. Looking back at the isInAny function we defined there, here’s how we’d use a partially applied function instead of a named helper function or a lambda:

-- file: ch04/Partial.hs
isInAny3 needle haystack = any (isInfixOf needle) haystack

Here, the expression isInfixOf needle is the partially applied function. We’re taking the function isInfixOf and fixing its first argument to be the needle variable from our parameter list. This gives us a partially applied function that has exactly the same type and behavior as the helper and lambda in our earlier definitions.

Partial function application is named currying, after the logician Haskell Curry (for whom the Haskell language is named).

As another example of currying in use, let’s return to the list-summing function we wrote in The Left Fold:

-- file: ch04/Sum.hs
niceSum :: [Integer] -> Integer
niceSum xs = foldl (+) 0 xs

We don’t need to fully apply foldl; we can omit the list xs from both the parameter list and the parameters to foldl, and we’ll end up with a more compact function that has the same type:

-- file: ch04/Sum.hs
nicerSum :: [Integer] -> Integer
nicerSum = foldl (+) 0


Haskell provides a handy notational shortcut to let us write a partially applied function in infix style. If we enclose an operator in parentheses, we can supply its left or right argument inside the parentheses to get a partially applied function. This kind of partial application is called a section:

ghci> (1+) 2
ghci> map (*3) [24,36]
ghci> map (2^) [3,5,7,9]

If we provide the left argument inside the section, then calling the resulting function with one argument supplies the operator’s right argument, and vice versa.

Recall that we can wrap a function name in backquotes to use it as an infix operator. This lets us use sections with functions:

ghci> :type (`elem` ['a'..'z'])
(`elem` ['a'..'z']) :: Char -> Bool

The preceding definition fixes elem’s second argument, giving us a function that checks to see whether its argument is a lowercase letter:

ghci> (`elem` ['a'..'z']) 'f'

Using this as an argument to all, we get a function that checks an entire string to see if it’s all lowercase:

ghci> all (`elem` ['a'..'z']) "Frobozz"

If we use this style, we can further improve the readability of our earlier isInAny3 function:

-- file: ch04/Partial.hs
isInAny4 needle haystack = any (needle `isInfixOf`) haystack

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